The present invention relates to a retardation film, manufacturing method thereof, polarizing plate and liquid crystal display apparatus, particularly to a retardation film with its curl having been improved for suitable use in a polarizing plate, the manufacturing method thereof, the polarizing plate and liquid crystal display apparatus using this retardation film.
The liquid crystal display apparatus is characterized by low voltage and low power consumption. It can be directly coupled to an IC circuit, and can be built into a thin configuration. Because of these advantages, the liquid crystal display apparatus has an extensive use as a display for word processors or personal computers. This liquid crystal display apparatus is basically structured in such a way that a polarizing plate is provided on one or both surfaces of a liquid crystal cell. The polarizing plate allows the passage of light only on the plane of polarization in a certain direction. In the liquid crystal image display apparatus, the polarizing plate plays a major role of ensuring that a change in orientation due to electric field is visible. The performance of the liquid crystal display apparatus heavily depends on the performance of the polarizing plate.
The polarizing plate is commonly structured as follows: For example, a protective film made of such a cellulose ester film as a cellulose triacetate film and cellulose acetate propionate film is bonded on one or both surfaces of the polarizer made up of polyvinyl alcohol film has been subjected to uniaxial drawing and iodine dyeing, or the retardation plate is further bonded on the aforementioned protective film through an additive as polyvinyl alcohol. Further, this polarizing plate is bonded to the liquid crystal cell through the adhesive layer.
In recent years, there is a tendency for the growing size of a liquid crystal display apparatus and the reducing thickness of the polarizing plate used therein. One of the big problems in the process of manufacturing is concerned with the absolute value for the size of curl when the polarizing plate is bonded on the glass of a liquid crystal cell, i.e. the height of the end of the cut polarizing plate raised from the horizontal plane or the height of the raised central portion of the cut polarizing plate. If the curl occurring to the aforementioned polarizing plate is not properly adjusted, when the polarizing plate is bonded to the panel, the plate will be raised or will be impregnated with bubbles. Alternatively, the angle of the plate will be misaligned. If such a problem has occurred, the polarizing plate having been bonded must be removed. This will take much time and labor. Alternatively, this problem may lead to the failure to use the panel per se, and hence a still bigger problem of reduced yield. This problem is serious especially when a thin film polarizing plate is manufactured.
An electric field control birefringent mode is often used in a large-sized liquid crystal display apparatus. In the VA or OCB system, the retardation plate is used in some cases. When the retardation plate is employed, a retardation plate can be bonded on a conventional polarizing plate. However, when this method is used, there is a reduction of the yield due to bonding error. Further, it is often very difficult to control the curl of the final polarizing plate product due to the lack of direct bondage with the polarizing film. To solve this problem, the function of the polarizing plate protective film is built into the retardation plate, which is bonded directly onto the polarizer. This art is effective in reducing the thickness of the polarizing plate.
The following arts are disclosed to control the curl of the polarizing plate: One is the art of controlling the amount of the plasticizer on both surfaces of the polarizing plate protective film to prevent a curl from occurring to the polarizing plate (refer Japanese Non-Examined Patent Publication 2002-258049). The other is the art of ensuring that the curls of the polarizing plate protective films bonded on both surfaces of the polarizer will be oriented opposite to each other (refer Japanese Non-Examined Patent Publication 2002-258050 and Japanese Non-Examined Patent Publication 2004-184809). However, these techniques are not sufficient to control the curl of the polarizing plate used in the manufacture of a large-sized liquid crystal display apparatus.